I wouldn't be TOO scared of overflow boxes. As long as they're made properly, they shouldn't lose syphon. I was looking at them last week, and also looking at what it would take to build my own.... and it's really a simple concept.
Look at it this way... this is what a basic overflow box should do:
Now, the left side is the side that's in the tank. Water flows through the weirs at the top of that into the cup. The outer cup would have to be filled by hand in order to prime it. To prime it, you just have to have some way to remove all the air from that hose so it maintains the same water level on both sides of it.... same concept as how you can submerge a cup, turn it upside down, and lift it out of the water, keeping water in it until it breaks the surface. As long as both sides stay underwater at all times, that hose is gonna stay filled, and both sides stay even.
Now, when it works correctly, the water level of the tank is higher than the water level of that divider near the bulkhead. Water comes into the cup, raises the water level through the hose, and it spills over into the bulkhead area and down into your sump. It gets pumped back up, spills over, repeats the process. No problem.
If the power goes out, the water is no longer being pumped up. The water level in the tank will eventually drop below the level of the overflow, so no water is going in it anymore. The two sides will even out at the level that the divider near the bulkhead is, and both ends of the tube stay submerged, and thus water stays in the tube. Your sump has to be able to handle the few gallons of water that are gonna come in that way but not be pumped back out yet..... and in that aspect it's not different than a drilled overflow.
All in all, if you can drill your tank properly, by all means, do it. BUT... don't be scared of those overflow boxes. As long as they're properly built, I can't see why they'd ever fail. I'd be FAR more scared of cracking the glass on a tank that big trying to drill it than I would of overflow box failure.
